Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in times on the order of milliseconds under the influence of an applied magnetic field. The utility of these materials is that suitably configured electromechanical actuators that use a MR fluid can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable power train and engine mounts, and in numerous electronically controlled force/torque transfer (clutch) devices. The MR mounts are controllable mounts that can provide vibration damping over a wide range of frequencies. In engine and power train mount applications, for example, a MR fluid is used in place of conventional hydraulic fluid, and a coil assembly replaces the orifice track used in hydraulic mounts. FIG. 1 depicts in cross-sectional view a typical MR fluid mount 10 having a coil assembly 12 incorporated into an orifice plate assembly 14, which separates an upper chamber 16 from a lower chamber 18 and contains an annular flow passage for the MR fluid 20. The coil assembly 12 can be energized to generate a magnetic field across the annular flow path of the MR fluid 20 as it flows between the upper and lower chambers 16, 18. These chambers 16, 18 are typically made of natural rubber formulations, with or without metallic inserts. The strength of the magnetic field can be varied to provide variable damping.
MR fluids are non-colloidal suspensions of finely divided low coercivity magnetizable solids, such as iron, nickel, cobalt and their magnetic alloys dispersed in a base carrier liquid. MR mounts have typically been made with glycol-based carrier fluids. These fluids have the advantages of being inexpensive and compatible with natural rubber. However, durability testing with these fluids has been unsuccessful because the fluids settle appreciably under durability test conditions. Because MR fluids contain non-colloidal solid particles that are often 7–8 times more dense than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor during use.
Silicone-based MR fluids have also been tested for use in MR mounts. However, these fluids also exhibit severe settling, as depicted in FIG. 2. FIG. 2 shows the variation of on-state and off-state forces for MR mounts using a silicone-based MR fluid over the course of durability testing. The MR fluid contained 20 wt. % carbonyl iron particles dispersed in silicone oil having a viscosity of 50 cSt, 12 wt. % surface treated fumed silica (Cab-O-Sil® TS-720 from Cabot Corporation), and no additives. It can be seen from FIG. 2 that these mounts exhibit a precipitous drop in the on-state force after a few hours of durability testing. One commercially available silicone-based MR fluid that is specifically recommended by the manufacturer for use in natural rubber mounts, specifically Lord MR Fluid No. MRF336AG, was also tested and exhibited severe settling when exposed to the natural rubber mount components during component testing.
The magnetizable particles are kept in suspension by dispersing a thickening agent, such as fumed or precipitated silica, into the formulation. Silica stabilizes the MR fluid by forming a network through hydrogen bonding between silica particles. This network breaks down under shear and reforms upon cessation of shear to keep the magnetizable particles suspended while exhibiting low viscosity under shear. Fumed silica is preferred as a thickener, and is typically surface treated. Despite the presence of the thickeners in the MR fluids, severe settling is occurring during durability testing of the MR fluids in natural rubber-based mounts.
There is thus a need to develop a MR fluid formulation that is compatible with natural rubber and is resistant to settling in devices where the fluid is in contact with natural rubber.